Mössbauer study of the site preference for Fe substitutions in some YBa2Cu3O7-ycompounds

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Abstract
YBa2Cu3O7-y compounds with 0.5% of the Cu atoms replaced by 57Fe and with y = 0.15, 0.48, 0.96 and 1.01 have been synthesized. X-ray diffraction showed that the structures are orthorhombic for y < 0.50, tetragonal for larger y and that the volume of the unit cell increases with y. The orthorhombic compounds are superconducting with critical temperatures Tc = 90.7 K and 49 K for y = 0.15 and 0.48 respectively. The tetragonal compounds are semiconducting with band gaps ΔE ≈ 0.15eV for y ≈ 1.0. The Mössbauer spectra give some hints to the site preference of the iron atoms from which we suggest that more than 84% of the Fe atoms substitute for Cu(1) and the rest for Cu(2). Charge states of +3 and +4 for the Fe(2) and the Fe(1) species are consistent with the values of the hyperfine parameters.